U.S. Pat. No. 3,861,470 and other related U.S. patents cited therein, all of which are owned by the same corporate entity which owns the subject invention, are directed to a blowout preventer method and apparatus which utilize a ball-shaped valve that opens and closes automatically in response to changes in pressure differential between the wellbore and the annulus between the drill string and the wall of the drill hole. When the pressure differential is lost a ball element is automatically rotated shut. After conditions return to normal a spring or other pressure exerting means operates to rotate the ball valve back to its open position.
Although this blowout preventor valve proved effective for its intended purpose, it has the disadvantage that well tools cannot be run below the valve when the ball is closed.
Further, with ball valves of the types described above the internal components inherently have a relatively small bore in the valve which causes a pressure drop across the valve and requires a higher pump pressure for effective operation.